Polyoxometalate Chemistry From Topology via Self-Assembly to Applications

Heteronuclear hexametalates [XM′ M₅O₁₈]ⁿ⁻(M = Mo, W),including the first examples of Zr and Hf derivatives, have been prepared by hydrolytic aggregation in non-aqueous media, enabling the reactivity of alkoxide surface groups (X = OM_e, Oprⁱ) to be investigated. Organoimido derivatives result from reactions between [Mo₆O₁₉]²⁻and organic isocyanates or aromatic amines at elevated temperatures. In studies of vanadate systems we have achieved the quantitative conversion of [H₃V₁₀O₂₈]³⁻ to [V₁₃O₃₄]³⁻ under ambient conditions and the synthesis of a range of new vanadophosphonates. The potential of non-aqueous reductive aggregation for rational polyoxometalate assembly has been demonstrated by the synthesis of [PV₂Mo₁₂O₄₂]³⁻ from [PMo₁₂O₄₀]³⁻ and [VOCl₃(dme)]. In the first examples of controlled polyoxometalate halogenation, the hexabromo species [PW₉O₂₈Br₆]³⁻ has been obtained from [NaPW₁₁O₃₉]⁶⁻ and [PW₉O₃₄]⁹⁻ by treatment with C₂O₂Br₂ or SOBr₂. The structure of this anion features a fully brominated face which provides opportunities for further derivatisation.

This contribution will focus on the functionalization of polyoxometalates with multiply bonded ligands, notably nitrosyl, imido and cyclopentadienyl ligands. The first part will define the scope of the different synthetic methodologies, i.e. net [2+2] reactions with Mo=O bonds, condensation-type reactions via a-hydrogen, and self-assembly reactions via the displacement of labile ligands, e.g. halide or solvent, from appropriate metal complexes. Selected examples will be presented and the eventual complications, e.g. hydrolysis or reduction, will be discussed. Special attention will be paid to the reactivity of phosphonium ylides towards polyoxomolybdates which contrasts that of phosphinimines.

The second part will show that functionalization may provide fine tuning of the electronic properties of the parent anion. Representative examples include the activation of surface oxygen atoms, as demonstrated by methylation of Lindqvist-type anions, and the stabilization of specific compounds, e.g. [M₄O₁₃(OMe)₄{Mo(NO)}]³⁻ (M = Mo or W), and [Mo₁₀O_31−x(OMe)_x(NO)]^z−, which display their own, interesting, chemistry. Furthermore, NMR and electrochemical data underscore some electronic communication between the attached ligand and the polyoxometalate moiety: a clear example is provided by the series [Mo₆O₁₈(NC₆H_4−p−X)]²⁻ where ⁹⁵Mo and ¹⁴N chemical shifts and reduction potentials correlate with the Hammett constant of the substituent.

Different strategies of synthesis were developed to introduce sulfur atoms in a polyoxometalate framework. Every synthetic route provides new and specific sulfur-containing compounds, characterized by single crystal X-ray diffraction and multinuclear NMR spectroscopy in solution. The first investigations based on conventional routes of synthesis give predictable oxo-thio Keggin-like clusters while an original strategy, based on the acid-base self-condensation of an oxo-thio building block is the origin of a new generation of polymetalates. Finally, under hydrothermal conditions, successive replacements of sulfur atoms by oxygen atoms take place and unexpected molecular associations between fully oxygenated saturated Keggin anions were obtained.

The presentation focusses on integrated oxometal clusters containing fac-{M(CO)₃}⁺ (M = Mn, Re), {Rh(η⁵-C₅Me₅₎²⁺ or {Ru(η⁶-arene)}²⁺ organometallic subunits and oxo(alkoxo)molybdenum or tungsten subunits. The discussion adresses the following questions: i) structural relationships, ii) structural preferences, and iii) stereochemical non-rigidity. The molecular structures of the organometallic oxometal clusters are discussed in connection with those of previously reported polyoxo(alkoxo)metalates and organometallic clusters. The apparent structural relationships within these clusters underscore the electronic connection between d⁶-fac-{ML₃} (M = Mn, Re, Ru, Rh,) and d⁰-fac-{MO₃} or d⁰-fac-{MO₂(OR)}⁺ (M = Mo, W) units.

In polyoxometalate chemistry, a large variety of clusters can be formed by linking together metal-oxide building blocks, including tetrahedra, octahedra, and even pentagonal units with C₅ symmetry. Correspondingly, it is possible to construct spherically shaped polyoxometalates with icosahedral symmetry and predetermined sizes by connecting those pentagonal units using appropriate linkers. Using tools from discrete mathematics, the resulting molecular architectures can be investigated and the basic geometric/topological principles governing their construction can be elucidated.

The enormous potential of polyoxometalates in fields of catalytic, technical or medical applications is dependent on the synthesis and structural characterization of new heteropolytungstates and -molybdates. As we are aware of the necessity to suggest new models for catalytic processes we put emphasis on the detailed characterization of so far unknown heteropolyoxoanions. Emphasis is given to a comprehensive investigation of the structures of polytungstates formed by Keggin derived fragments. The following chapter deals with syntheses and structures of novel selenium- and tellurium-substituted heteropolyoxometalates. Hitherto, only a few polyoxometalates containing Se or Te are reported; this field is hardly investigated. Giving results of the structural characterization of [Mn(H ₂O)) ₂OSe₆W₂₄O₉₄]¹⁸⁻ (6), [Fe ₄ Se ₈ W ₃₆ O ₁₃₆ (OH) ₆]²⁴⁻, (7) and the [Na ₉ Se ₁₃(VO ₄)₃ V ₂₃ Mo ₁₉ O ₁₅₁]²¹⁻ -anion (8), three new structural arrangements of macro- heteropolyanions are described. Furthermore, we focus on sandwich-like polyoxotungstates consisting of defect Keggin fragments like B-XW ₉ O ₃₃. In the anion [Se ₂ V ₂ W ₁₆ O ₆₀]⁸⁻ lacunary subunits are directly linked together whereas in the other ten compounds (1–3, 5, 9– 14) connection is reached by oxygen atoms coordinated to transition metal atoms. All crystal structures were determined by single crystal X-ray methods.

The vibrational spectra of the heteropoly acids can be conveniently regarded as composed of contributions from the polyoxometalate anion (the primary structure) and from the water of crystallisation and hydrated protons (the secondary structure). Following a brief general survey of vibrational spectra of hydrogen-bonded systems, the spectra of water and hydrated protons in crystalline solids are reviewed. The vibrational spectra of the primary structure of heteropoly acids (the Keggin anion) are described and the observed spectra of the secondary structures of highly hydrated, hexahydrated and dehydrated heteropoly acids are discussed in relation to the spectra expected for protons in different environments.

In spite of thorough investigations, the most debated issue of bond-stretch isomerism has remained elusive up to now in transition metal chemistry. DFT calculations are reported on the reduced Keggin oxothio heteropolyanions γ-[SiW ₁₀ Mo ₂ S ₂ O ₃₈]⁶⁻ (1), γ-[SiW ₁₂ S ₂ O ₃₈]⁶⁻ (2), γ[SiW ₁₀ Mo ₂ O ₄₀]⁶⁻ (3) and γ-[SiW ₁₂ O ₄₀]⁶⁻ (4), obtained from the stereospecific reaction between a preformed [M ^v ₂ X ₂ O ₂]²⁺ cation (M=Mo,W; X=S,O) and a polyvacant γ-Keggin anion. The calculations show that those four clusters display the distinctive signature of bond-stretch isomerism, namely the presence of a double minimum on their potential energy surface depending on a single metal-metal distance. The energy minima are assigned to the localisation of the metal electron pair into the cationic moiety giving rise to a metal-metal bond, and to its transfer to the γ-Keggin core, respectively. The energy barriers separating the two minima do not exceed 6kcal.mol ⁻¹ , which precludes a physical separation of the isomers. At variance with small clusters containing a limited number of metal atoms, supramolecules made of the assembly of several organometallic/inorganic fragments could be well suited to bond-stretch isomerism due to the possibility of intramolecular electron transfers with structural consequences similar to those of standard oxido-reduction.

The results of the quantum-chemical DFT (density functional theory) studies of the electron transfer between the substituted transition metal and the polyoxoanion addenda atoms are presented. This work is motivated by the experimental research on the catalytic activity of the iron-substituted 12-molybdophosphoric acid and its salts in the oxidation of alcanes. Two cases have been considered corresponding to two experimental situations extensively studied by Mössbauer spectroscopy:

New ¹⁷O NMR, potentiometry and ESI-MS measurements on aqueous peroxomolybdates and peroxotungstates reveal many new species, including diperoxo hepta-anions and also confirm other proposals based on potentiometry alone, such as a monoperoxo monomer. They also show the presence of many anions previously identified only in the solid state, including both forms of the Mo ₂ (peroxide) ₄ anion. Comparison with recent work on peroxovanadates and peroxoniobates shows a marked preference in all cases for each metal atom to be coordinated to two peroxo ligands. Peroxotungstates, like tungstates themselves, are generally more complex than peroxovanadates and -molybdates.

Peroxomolybdates have shown to be efficient selective agents in the degradation of lignin in non-chlorine based bleach processes of kraft pulp. Furthermore, the process can be improved when anions such as phosphate are present. To clarify the chemistry in aqueous solution, fundamental speciation studies of relevant systems have been made under conditions similar to those in the bleaching step.

In this article the equilibrium speciation in the system p H ⁺ + q MoO ₄ ²⁻ ⇆ (H ⁺) _p (MoO ₄ ²⁻ ) _q in 0.300 M Na ₂(SO ₄) medium at 25 °C has been studied using potentiometric data in the range 2.5 <- pH <- 6.0, 1.25 <- Mo _tot /mM <- 20. The speciation was found to consist of the monomers MoO ₄ ²⁻ (0,1), HMoO ₄ ⁻ (1,1), H ₂ MoO ₄(2,1) and the heptamers Mo ₇ O ₂₄ ⁶⁻ (8,7), HMo ₇ O ₂₄ ⁵⁻ (9,7), H ₂ Mo ₇ O ₂₄ ⁴⁻ (10,7) and H ₃ Mo ₇ O ₂₄ ³⁻ (11,7) (numbers in parentheses refer to the values of p and q in the general reaction above). The following formation constants and 3σ were obtained: log β _1,1 = 3.40 ± 0.10, log β _2,1 = 7.79 ± 0.06 (p K _a = 4.39), log β _8,7 = 52.43 ± 0.04, log β _9,7 = 57.42 ± 0.03 (p K _a = 4.99), log β _10,7 = 61.24 ± 0.04 (p K _a = 3.82) and log β _11,7 = 63.90 ± 0.10 (p K _a = 2.66). The p K _a value for HSO ₄ ⁻ was determined to 1.27 ± 0.01. The effects of different ionic media on this system are discussed.

Finally, this article presents some preliminary results of the ongoing speciation studies in the H ⁺ - MoO ₄ ²⁻ - H ₂ O ₂ and H ₊ - MoO ₄ ²⁻ - H ₂ O ₂ - H ₂ PO ₄ ⁻ systems.

¹H NMR spectroscopy was used to study various ligands coordinated to some paramagnetic polyoxometalates (POMs). Pure signals of the complexes were observed, which indicates that ligand exchange is slow on the NMR time scale. Mono- and diprotonated species of [CoW ₁₁ CoO ₃₉ ] ⁸⁻ and [SiW ₉ Cu ₃ O ₃₇ ] ¹⁰⁻ were detected from the spectra of pyridine coordinated to these POMs. 2-Aminopyridine binds to [SiW ₁₁ CoO ₃₉ ] ⁶⁻ (SiW ₁₁ Co) whereas 2-methylpyridine does not. This indicates that hydrogen bonding between the amine group and a bridging oxygen atom on SiW ₁₁ Co plays an important role in complex formation. 4- Aminopyrimidine forms two linkage isomers, a and b, binding to SiW ₁₁ Co via N(1) and N(3), respectively. The relative amount of isomer b increases, when D₂O is replaced by DMF, indicating that hydrogen bonding between the amine group and SiW ₁₁ Co is more favorable in DMF than in D ₂ O. 3,3-Dimethylpiperidine undergoes rapid chair-chair interconversion at room temperature. When it is coordinated to SiW ₁₁ Co in D ₂ O the conformation is frozen even at room temperature. When DMSO is added to a D ₂ O solution, the spectral change indicates that another conformation is stabilized in DMSO.

The exploitation of mixed heteronuclear rare-earth-element-containing polyoxometalates to probe the multipolar nature of heteronuclear rare-earth interactions is imaginative. It appears that polyoxometallolanthanoates are ideal for this type of investigation. Three structural types of Tb³⁺/Eu³⁺ heterolanthanide-multinuclear polyoxometalates, K₁₅H₃[Tb_1.4Eu_1.6(H₂O)₃(SbW₉O₃₃)(W₅O₁₈)₃]•25.5H₂O, Na₇H₁₉[Tb_4.3Eu_1.7O₂(OH)₆(H₂O)₆Al₂(Nb₆O₁₉)₅]•47H₂O, and [NH₄]₁₂H₂[Tb_3.1Eu_0.9(MoO₄)(H₂O)₁₆(Mo₇O₂₄)₄]•13H₂O are studied by crystal structures, emission and excitation spectra, and emission decay dynamics. The excitation of the Tb^{3+ 7}F₆→⁵D₄ transitions produces not only the emission lines of Tb³⁺, but also those of Eu³⁺, accompanied by nonexponential rise and decay curves of the emission from Tb³⁺ and Eu³⁺. There is no significant exchange interaction between the lanthanide ions, as a result of the coordination of aqua and/or hydroxo ligands to the lanthanide ions. The mechanism of the Tb³⁺→Eu³⁺ energy transfer is identified as a Forster-Dexter-type energy transfer from Tb³⁺ (donor) to Eu³⁺ (acceptor). The nearest-neighbor energy-transfer rates by electric dipole-dipole interactions between a Tb-Eu pair at 4.2K are estimated to be 4.5 × 10⁴, 4.7 × 10⁵, and 4.9 x 10³ s⁻¹ and the critical radii at 4.2 K are 10.3, 10.0, and 6.17 Å for K₁₅H₃[Tb_1.4Eu_1.6(H₂O)₃(SbW₉O₃₃)(W₅O₁₈)₃]•25.5H₂O (with Tb-Eu separation of 5.05 Å), Na₇H₁₉[Tb_4.3Eu_1.7O₂(OH)₆(H₂O)₆Al₂(Nb₆O₁₉)₅]•47H₂O (with 3.76 Å separation), and [NH₄]₁₂H₂[Tb_3.1Eu_0.9(MoO₄)(H₂O)₁₆(Mo₇O₂₄)₄]•13H₂O (with 6.17 Å separation), respectively. The low symmetry (C _s or C ₁ ) of the LnO ₈ (Ln=Tb and Eu) coordination polyhedra allows the nonvanishing electric-dipole transition probability for the ⁷ F _J ↔; ⁵ D ₀ (J=0,l) transitions which leads to a faster transfer rate at high temperatures. The photoexcitation of the host lattices (tungstate, niobate, and molybdate) induced the energy transfer from the oxygen-to-metal charge-transfer {O→M(=Nb, Mo, W) lmct} triplet states to Tb ³⁺ ( ⁷ F ₆ → ⁵ D ₄ ) and Eu ³⁺ ( ⁷ F _0,1,2 → ⁵ D _0,1 ). In the case of [NH ₄ ] ₁₂ H ₂ [Tb _3.1 Eu _0.9 (MoO ₄ )(H ₂ O) ₁₆ (Mo ₇ O ₂₄ ) ₄ ]•13H ₂ O this transfer is not complete and the O→Mo lmct triplet emission of molybdates is observed to provide the rate constant for the energy transfer to Tb³⁺/Eu³⁺ sites with 4.4 × 10⁶ s⁻¹

The potentialities of the use of polyoxometalates as inorganic components in conducting and/or magnetic organic/inorganic hybrid molecular materials are illustrated with few examples of their chemical and electrochemical assemblies with organic donors derived from TTF and BEDT-TTF, nitronyl nitroxide and metallocenium radical cations.

The present article highlights recent results and provide a perspective of the interest of polyoxometalates as inorganic component of molecular materials with active physical properties. Three different aspects will be presented: i) The interest of the magnetic and mixed valence clusters provided by polyoxometalate chemistry in molecular magnetism; ii) The use of these inorganic anions as magnetic component of crystalline conducting materials based on organic donor molecules; iii) The construction of well-organized films of polyoxometalate monolayers by using the Langmuir-Blodgett technique.

Transition metal oxide clusters and their derivatives, which are important in such diverse fields as analytical chemistry, biochemical and geochemical processes, chemical sensing, catalysis, materials science, and medicine offer an unmatched variety of attractive building block units for the design and development of new materials whose properties could possibly be correlated with their constituent units at the molecular level. For example, these clusters may provide structural motifs for the rational synthesis of new metal oxide based catalysts and novel surfaces. Although the technique of bringing suitable transition metal oxide units together to generate new chemical systems with desirable properties remains underdeveloped, recent progress made in preparing new framework materials are encouraging. By adopting essentially one-pot synthetic approach, a series of novel framework materials, composed of well defined vanadium oxide clusters {V ₁₈ O ₄₂ (XO ₄ )}, of general formulation- (Cat) _m [M _a L ₁₂ V ₁₈ H _b O ₄₂ (XO ₄ )] ⁿ⁻ .cH₂O. (Cat = Li⁺, N₂H₅⁺; m = 0, 2, 6; M = Mn, Fe, Co, Ni, Zn, Cd, etc.; a = 3; b = 0, 6; X = V, S; L = H₂O; n = 0, 2, 6; c = 24, 30) have been prepared and characterized. These are reviewed with reference to the synthesis, structure and physicochemical properties of the newly prepared representative framework solids.

The chemistry of polyoxomolybdate anions may be exploited in a building block approach to the synthesis of solid state oxide materials. One strategy controls the oxide microstructure by introducing a secondary metal-ligand subunit as a charge-compensating, space-filling and structure-directing component. The resulting organic-inorganic hybrid materials are representative of solid state coordination chemistry in which the oxide microstructure reflects the geometry of the ligand, the coordination preferences of the secondary metal center, and the synergistic interaction of the coordination complex cation with the polyoxomolybdate component.

Giant molybdenum oxide-based clusters in which pentagonal {(Mo)Mo₅} units are linked by a large number of paramagnetic centers like 30 Fe^III or 20 VO²⁺ show novel and unusual types of spin topologies: here we present an Fe₃₀ icosidodecahedron and a magnetic ring-shaped band built up by 10 {V ^IV ₃} triangles sharing vertices.

α-Phosphovanadotungstate monolayers are formed on Ag(111) by reaction of the surface with α-PVW₁₁O₄₀⁴⁻in acidic aqueous solution. The ordered domains formed on smooth Ag(111) terraces are extremely large, on the average over 1,000 nm²in area. Comparison of scanning tunneling microscopy (STM) images of the Ag(111) surface during immersion in solutions containing α-PVW₁₁O₄₀⁴⁻and α-PVW₁₁O₄₀⁵⁻ show that the former, oxidized form is required for the formation of ordered domains. A combination of UV-visible and ICP-atomic absorption spectroscopy confirmed that α-PVW₁₁O₄₀⁴⁻is an effective etchant for the Ag(111) surface.

The Keggin-type di-iron-substituted silicotungstate, γ-SiW₁₀{Fe(OH₂)}₂O₃₈⁶⁻, can catalyze the selective oxidation of various alkanes including methane with hydrogen peroxide. The tetrabutylammonium salt catalytically oxidized cyclohexane, n-hexane, n-pentane, and adamantane in acetonitrile. Even lower alkanes such as methane, ethane, propane, and n-butane were catalytically oxidized. It is remarkable that the efficiency of hydrogen peroxide utilization to oxygenated products reached up to ca. 100% for the oxidation of cyclohexane and adamantane. The efficiency and activity for the utilization of hydrogen peroxide greatly depended on the iron centers and di-iron-substituted γ-SiW₁₀{Fe(OH₂)}₂O₃₈⁶⁻ showed the highest efficiency of hydrogen peroxide utilization and conversion. Such a structure dependency of the catalysis is significant and the remarkable catalytic performance of di-iron-substituted polyoxometalate may be related to the catalysis by methane monooxygenase. It was also demonstrated that the water-soluble potassium salt catalytically oxidized the lower alkanes with hydrogen peroxide in water.

Two major reaction modes have been perceived for the catalytic activity of polyoxometalates in oxidation reactions. In one case, the catalytic cycle has been described by the division of the reaction into two stages. First, the substrate is oxidized by consecutive electron and proton transfer by the polyoxometalate in the oxidized form to yield the product and the reduced polyoxometalate catalyst. The reduced polyoxometalate catalyst is then reoxidized, importantly by molecular oxygen to form water, in the second and possibly separate stage completing the catalytic cycle. The polyoxometalates often most effective in this reaction are the phosphovanadomolybdates of the Keggin structure, H _3+x PV _x Mo _12−x O ₄₀ (x = 1 − 6, especially x = 2). Now, new investigations of the reactivity of PV ₂ Mo ₁₀ O ₄₀ ⁵⁻ with aldehydes and quinones enables the differentiation between the reactivity of the five inseparable isomers of α-H ₅ PV ₂ Mo ₁₀ O ₄₀ using ³¹ P NMR and ESR spectroscopy, and UV-vis absorption-time profiles. The 1,11 isomer with vanadium in distal positions is the most abundant, although the isomers with vanadium in vicinal positions appeared to be the most kinetically viable. For example, alkane/aldehyde/O ₂ oxidizing systems were found to be quite effective and selective for oxidation of alkanes to ketones. Further studies of PV ₂ Mo ₁₀ O ₄₀ ⁵⁻ - quinone interactions has shown the formation of semiquinone intermediates. The later are active in the dehydrogenation of benzylic and allylic alcohols to aldehydes and can be used as models for the reactivity of PV ₂ Mo ₁₀ O ₄₀ ⁵⁻ on carbon supports. The second reaction type views the oxidation catalyzed by the polyoxometalate as an interaction with a primary oxidant. This interaction yields an activated catalyst intermediate eg a peroxo, hydroperoxo or oxo species which can be used to oxidize the organic substrate. In this mode, one can consider reaction at a transition metal substituted position within the polyoxometalate. Here the polyoxometalate acts as an “inorganic ligand” for transition metals such as cobalt, manganese, ruthenium, etc. In mechanistic scenarios for such reactions, the catalytically active site is a tetragonally (pyrimidal) oxo coordinated transition metal while the polyoxometalate as a whole functions as a ligand with a strong capacity for accepting electrons. In this last group of oxidation reactions the actual reaction mechanism certainly varies as a function of the transition metal and oxidant, but can be conceived to take place via a general intermediate “transition metal - oxidant” species. The ruthenium substituted “sandwich” type polyoxometalate, [WZnRu ^III 2(ZnW ₉ O ₃₄ )₂] ¹¹⁻ , has been shown to activate molecular oxygen in a dioxygenase type mechanism, and selectively catalyze thereby a) the hydroxylation of alkanes at the tertiary carbon position and b) the stereoselective epoxidation of alkenes. For comparison, catalytic oxidation of a novel ruthenium substituted polyoxometalate, [Ru^II(H₂O)W₁₇O₅₅F₆NaH₂]⁹⁻, in similar reactions appears to occur by a metal catalyzed autooxidation.

Following an introductory overview of the 8 newer classes of polyoxoanion-based catalysts in the last 20 years, as well as the 3 subclasses of polyoxoanion-based catalysts investigated by the Finke Group, highlights are presented of the research which has led to two of the 8 newer classes of polyoxoanion-based catalysts: polyoxonion-stabilized transition metal nanocluster “soluble heterogeneous catalysts” and polyoxoanion-based precatalysts for catechol dioxygenase activity. In both cases, the resultant catalysts show record catalytic lifetime as well as high catalytic activity. An introduction to dioxygenase catalysis, including some of the key goals in the area, is also provided. The highlights of a recently reported, polyoxoanion •Fe^II-based stiochiometric catechol extradiol dioxygenase are then presented. Key findings from polyoxoanion-derived catechol dioxygenase catalysts are presented next, catalysts which exhibit record catalytic activity as well as record catalytic lifetime. A summary, emphasizing the 4 key components of research which have led to the polyoxoanion-stabilized nanocluster and polyoxoanion-derived dioxygenase catalysts, is also provided.

Heteropolytungstates play a dual role in ribosomal crystallography. Beside generating phases, one of them, (NH₄)₆(P₂W₁₈O₆₂)14H₂O, was found to be extremely useful in inducing post crystallization rearrangements. These led to a significant increase in the internal order of crystals of the small ribosomal subunits from Thermus thermophilus, manifested in a dramatic extension of the resolution of their diffraction patterns, from the initial 7–9 Å to 3 Å. The current 3.3 Å electron density map of this particle, constructed using phases obtained from this W cluster together with other metal compounds, shows the recognizable overall morphology of the small ribosomal subunit. Over 96% of the nucleotides were traced and the fold of all proteins was determined fully or partially. Specific sites were determined independently by covalently bound heavy atom clusters, among them the surface of two proteins and a functional center, the gate for mRNA binding. All tungsten-cluster sites detected in this map are located in close proximity to the proteins of the particle, in positions that may have an influence on the stability and the rigidity of this rather flexible ribosomal subunit.

Polyoxometalates (POM), at least PW₁₂O₄₀³⁻, SiW₁₂O₄₀⁴⁻ and W₁₀O₃₂⁴⁻, are effective photocatalysts for the mineralization of diversified organic pollutants, such as, lindane, cresol, phenol, chlorophenols and polychlorinated phenols, and chloroacetic acid. Key reactions are the formation of OH radicals, the high affinity of organic pollutants for POM, and the regeneration of catalyst by dioxygen. The mineralization (i.e. formation of CO₂, H₂O and inorganic anions) proceeds via several intermediates resulting from H-atom abstraction, hydroxylation and to a lesser extent dehalogenation. The breaking of the aromatic ring is followed by the formation of several saturated and unsaturated organic acids. In all cases so far, ethanoic acid has been detected. The formation of -CH₂ and -CH₃ groups from aromatic carbons (i.e. -CH groups) suggests that a reductive pathway accompanies the oxidation process. The overall photobehavior of POM resembles the highly publicized photodecomposition of organic pollutants by TiO₂.